After his "Russian spring" debacle, for Putin Syria seems to be the last chance to get his boots on the ground at least somewhere, to prove (to himself and to his supporters) that he is still a world player with a long reach and perhaps to exchange something in Donbas and Crimea for his Syrian (non)involvement.
Don't know if you heard this one:
http://www.rferl.org/content/ukraine.../27233898.html
According to Russian sources, the nickname Yatsenyuk used participating in Chechen war was ... ready? ... "Viking".
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